This doesn't mean anything as it stands. But consider interpreting the sense more broadly: it could mean something like "this might as well not be here" (e.g. the door exists, but does not fulfil its purpose), or "this is not real" (e.g. the door seems to signify something which isn't really true or doesn't really exist). Incidentally, "This Is Not Here" is an exhibit of Yoko Ono.
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BillySep 18 '11 at 13:00

I think it's some sort of paraphrase on the line "nothing is real" from Strawberry Fields Forever. It could literally mean "This is not here. You're not really looking at it." — as in, nothing around you is real, nothing is here. That's my take on it.

I think in the literal sense, as the boy said to Neo in The Matrix, "There is no spoon." To understand this, you must also understand reality without limitations. Everything is made of atoms, 10% matter / 90% space. Our reality manifests our condition to be truth when it's in reality an illusion. If you are awake this will make more sense; if you are still in a walking coma from your reality, wake up.